EP0532432A1 - Artificial kidney - Google Patents

Abstract

An artificial kidney comprises: - an exchanger (1) having two compartments (2, 3) separated by a semipermeable membrane (4), a first compartment (2) being connected to a circuit (5, 7) for extracorporeal circulation of the blood, which circuit can be connected to a patient (9), the second compartment (3) having an exit for the drainage of a used liquid, - means (20, 21) for introducing to the patient (9) through a drip, a drip liquid containing a substance (A), and - dosing means for dosing the substance (A) into the blood of the patient (9) at a determined concentration, these dosing means comprising means (22, 25) for adjusting the flow rate of the drip liquid as a function of the flow rate of used liquid. This kidney makes it possible to dose a substance, such as medication, glucose or bicarbonate, precisely into the blood of a patient subjected to a haemofiltration, haemodialysis or haemodiafiltration treatment. <IMAGE>

Description

The present invention relates to an artificial kidney which allows the determination of substances in the blood and which is also particularly suitable for the treatment of people who have temporarily lost the use of their kidneys following an accident or an operation. surgical.

These substances can be, for example, drugs (antibiotics, in particular), glucose, or certain electrolytes of the blood (potassium, magnesium, bicarbonate, in particular). In the following, the invention will be described in its application to the dosage of bicarbonate, but it goes without saying that this example of use, given purely by way of illustration, cannot be considered to be limiting.

We know that the kidneys, in addition to the functions of purifying plasma waste (urea, creatinine) and water excretion, play an important role in maintaining the acid-base balance of the internal environment, in particular by eliminating weak acids. (phophates, monosodium acids) and by producing ammonium salts.

In people who have lost the use of their kidneys, temporarily or permanently, this regulating mechanism no longer playing, there is an increase in the acidity of the internal environment (acidosis), that is to say also a decrease in pH of the blood serum, which moves towards 7 (the blood pH normally varies within very narrow limits which range from 7.35 to 7.45).

Conventionally, to compensate for the deficiency of the renal regulatory mechanism, action is taken on another regulating mechanism of the acid-base balance of the internal medium, which is constituted by the blood buffer systems, the main one of which comprises carbonic acid, as a weak acid, associated with its alkaline salt, bicarbonate. This is how, to fight a person's acidosis suffering from renal insufficiency, bicarbonate is passed into his blood, generally simultaneously with a session of purification of the blood by hemodiafiltration or by hemodialysis.

During a hemofiltration treatment, where the blood is purified by ultrafiltration of plasma water through a semipermeable membrane with simultaneous convective transfer of plasma waste, the bicarbonate supply is made by infusion of a bicarbonate solution of sodium.

During a hemodialysis treatment, where the blood is purified by diffusive transfer of plasma waste through a semi-permeable membrane whose face not bathed by blood is irrigated by a flow of dialysis liquid, the intake of Bicarbonate can be done in two ways, depending on whether the dialysis fluid contains bicarbonate or is free of it.

In the first case, the bicarbonate is supplied to the blood by diffusion through the semi-permeable membrane, from the dialysis liquid to the blood, the concentration of bicarbonate in the dialysis liquid being adjusted accordingly.

In the second case, as during hemofiltration treatment, a solution of sodium bicarbonate is infused into the patient in an amount sufficient to compensate for the diffusive (or convective, hemofiltration) losses that occur in the membrane exchanger as well as the deficit from which the patient suffers from acidosis.

The final bicarbonate concentration in the blood of a patient undergoing one of these treatments depends on the bicarbonate concentration of the infusion solution or dialysis fluid, their respective flow rates, and the flow of the patient's blood circulating in a circuit for extracorporeal circulation of blood comprising the membrane exchanger. Currently, with the exception of the concentration of the sodium bicarbonate solution, which is set by the manufacturer, these parameters are determined empirically by the doctor, based on regular blood pH measurements in the case of patients in a state of shock, whose blood is permanently dialyzed or ultrafiltered, or done after a treatment session and before the next session, in the case of patients who have definitively lost the use of their kidneys. As a result, the bicarbonate concentration in the patient's blood rarely corresponds exactly to the desired concentration.

An object of the invention is to provide an artificial kidney which makes it possible to act with precision on the acid-base balance of a patient suffering from renal insufficiency.

• un échangeur ayant deux compartiments séparés par une membrane semi-perméable, un premier compartiment étant relié à un circuit pour circulation extracorporelle de sang connectable à un patient, un second compartiment ayant une sortie pour l'évacuation d'un liquide usé,
• des moyens pour perfuser au patient un liquide de perfusion contenant une substance (A),

caractérisé en ce qu'il comprend des moyens de dosage pour ajuster la concentration de la substance (A) dans le sang du patient à une concentration souhaitée [A ]_DES. compte tenu du transfert diffusif et/ou convectif de la substance (A) au travers de la membrane de l'échangeur.According to the invention, to achieve this goal, an artificial kidney is provided comprising:

• an exchanger having two compartments separated by a semi-permeable membrane, a first compartment being connected to a circuit for extracorporeal circulation of blood connectable to a patient, a second compartment having an outlet for discharging a used liquid,
• means for infusing the patient with an infusion liquid containing a substance (A),

characterized in that it comprises dosing means for adjusting the concentration of the substance (A) in the patient's blood to a desired concentration [A] _DES . taking into account the diffusive and / or convective transfer of the substance (A) through the membrane of the exchanger.

According to a characteristic of the invention, the metering means comprise means for adjusting the flow rate of the perfusion liquid (Q _A ) as a function of the flow rate (Q _OUT ) of spent liquid.

ou par la formule : $${\text{Q}}_{\text{A}}\text{=}\frac{{\text{[A]}}_{\text{DES}}}{{\text{[A]}}_{\text{SOL}}}\text{x Cl}$$

dans lesquelles [A]_SOL est la concentration de la substance (A) dans la solution stérile et Cl est la clairance du rein artificiel pour la substance (A).According to another characteristic of the invention, the flow rate (Q _A ) of infusion of the sterile solution and the flow rate (Q _OUT ) of the used liquid are linked by the formula: $${\text{Q}}_{\text{AT}}\text{=}\frac{{\text{[AT]}}_{\text{OF}}}{{\text{[AT]}}_{\text{GROUND}}}{\text{x Q}}_{\text{OUT}}$$

or by the formula: $${\text{Q}}_{\text{AT}}\text{=}\frac{{\text{[AT]}}_{\text{OF}}}{{\text{[AT]}}_{\text{GROUND}}}\text{x Cl}$$

where [A] _SOL is the concentration of substance (A) in the sterile solution and Cl is the clearance of the artificial kidney for substance (A).

According to another characteristic of the invention, the substance (A) being bicarbonate, the artificial kidney comprises a source of dialysis / substitution liquid not containing bicarbonate connected to the circuit for extracorporeal circulation of blood and to an inlet of the second compartment exchanger, and occlusion means to either isolate the source, or allow the flow of dialysis liquid / substitution in the circuit for extracorporeal circulation of blood, or allow the flow of dialysis liquid / substitution in the second exchanger compartment.

According to yet another characteristic of the invention, the artificial kidney comprises means for taking stock of the liquid (s) introduced into the circuit for extracorporeal circulation of blood and of the used liquid (ultrafiltrate and / or spent dialysis fluid) flowing from the second compartment of the exchanger. These means for taking stock of liquids may include a balance for weighing a reservoir constituting a source of dialysis / substitution liquid and a reservoir for collecting used liquid, and a balance for weighing a reservoir constituting a source of liquid containing the substance. (AT).

Other characteristics and advantages of the invention will appear on reading the description which follows. Reference is made to the appended drawings in which:

Figure 1 is a simplified diagram of a first embodiment of the invention.

Figure 2 is a simplified diagram of a second embodiment of the invention.

The artificial kidney shown in Figure 1 comprises an exchanger 1 having two compartments 2, 3 separated by a semi-permeable membrane 4. The compartment 2 is connected to a circuit for extracorporeal circulation of blood comprising an upstream channel 5, on which is disposed a circulation pump 6, and a downstream pipe 7 equipped with a bubble trap 8. The pipes 5 and 7 are respectively provided at their free end with a needle or a connector for catheter, for the connection of the circuit for extracorporeal circulation of blood to a patient's vascular system 9.

A reservoir 10 of sterile dialysis / substitution liquid and devoid of bicarbonate is connected, via a common line portion 11 on which is disposed a circulation pump 12, to two lines 13, 14 respectively connected to the trap. bubbles 8 and at an entrance to the second compartment 3 of the exchanger 1. Occlusion means 15, 16, such as electromagnetic clamps, are provided respectively on the pipes 13, 14 to allow selective connection of the reservoir 10 to the exchanger 1 or to the circuit for extracorporeal circulation of blood .

A second reservoir 17 for spent liquid (ultrafiltrate and / or spent dialysate liquid) is connected to an outlet of the second compartment 3 of the exchanger 1, by a pipe 18 on which is disposed a pump 19 for extracting spent liquid. The pump 19 makes it possible to create a variable depression in the compartment 3 of the exchanger 1, that is to say also to vary the transmembrane pressure and, consequently, the ultrafiltration flow rate.

A third reservoir 20 of sterile sodium bicarbonate solution is connected to the bubble trap 8, by means of a pipe 21 on which a circulation pump 22 is arranged.

According to the invention, the artificial kidney shown in FIG. 1 comprises means for establishing a balance sheet (s) of liquid (s) infused (s) to patient 9 and of used fluids, to optionally program a desired weight loss by extraction of excess plasma water relative to the liquid (s) infused, and to adjust the bicarbonate concentration of the patient's plasma to a determined value. These means include a first balance 23 for weighing the reservoir 10 for dialysis / substitution liquid and the reservoir 17 for spent liquid, a second balance 24 for weighing the reservoir 20 for sodium bicarbonate solution, and a control unit 25 capable of receiving , as input signals, the information provided by the scales 23, 24, a setpoint Q _WL of desired weight loss rate, the value [HCO₃] _SOL of the bicarbonate concentration of the solution contained in the reservoir 20 and an instruction [HCO₃] _DES for the desired concentration of blood in bicarbonate. The control unit 25 is provided for controlling the pump 19 for extracting spent liquid, taking into account the desired weight loss rate Q _WL and the rate Q _IN imposed on the pump 12 for circulation of the dialysis / substitution liquid, and to control the pump 22 for infusing the bicarbonate solution taking into account the flow rate Q _OUT of the pump 19 for extracting spent liquid.

According to the invention, the control of the flow rate Q _HCO3 of the perfusion pump 22 to the flow rate Q _OUT of the extraction pump 19 can be defined, regardless of the type of treatment to which the patient is subjected, hemofiltration with or without infusion of substitution liquid, hemodialysis or hemodiafiltration, by the formula: $${\text{Q}}_{\text{HCO3}}{\text{= Q}}_{\text{OUT}}\text{x}\frac{{\text{[HCO₃]}}_{\text{OF}}}{{\text{[HCO₃]}}_{\text{GROUND}}}$$

où Cs est la concentration de la substance dans le sang et où Tr est la transmittance de la membrane vis à vis de cette substance.)The functioning of the artificial kidney which has just been described is as follows:
In hemofiltration mode without perfusion of substitution liquid, the clamps 15, 16 are closed, the pump 12 for circulation of the dialysis / substitution liquid is stopped and the pumps 19, 22 for extracting blood filtrate and perfusing baking solution rotate. The control unit 25 continuously adjusts the flow rate Q _OUT of the extraction pump 19, measured by means of the balance 23, so that this flow rate is permanently equal to the sum of the flow rate Q _WL of desired weight loss and of the Q _HCO3 rate of infusion of the bicarbonate solution, measured by means of the balance 24. The control unit 25 also continuously regulates the Q _HCO3 rate of the pump 22 of bicarbonate solution, as a function of the concentration of bicarbonate [HCO₃] _DES desired patient's blood, the concentration [HCO₃] _SOL of the solution contained in the reservoir 20, and the convective loss occurring in exchanger 1, which is equal to Q _OUT x [HCO₃] _BLD , [HCO₃] _BLD being the bicarbonate concentration of the patient's blood and the transmittance of the high permeability membranes used in hemofiltration being equal to 1 for the electrolytes of the blood (it is recalled that the general formula giving the mass flow rate Js of a substance crossing a membrane in function the volume flow Jv of plasma water is as follows: $$\text{Js = Jv x Tr x Cs}$$

where Cs is the concentration of the substance in the blood and where Tr is the transmittance of the membrane with respect to this substance.)

The enslavement of the bicarbonate perfusion pump 22 according to the formula (1) given above therefore makes it possible to gradually bring the patient's blood 9 to a state of equilibrium where the bicarbonate concentration is equal to [HCO₃] _DES .

In hemofiltration mode with infusion of substitution liquid, the clamp 16 is closed, the clamp 15 is open and the three pumps 12, 19, 22 rotate, the flow rate of the pump 12 being fixed by the operator at a constant value at the start treatment session. The functioning of the artificial kidney for this second mode of treatment does not differ from that which has just been described above, except that, to control the extraction pump 19, the control unit 25 takes into account the emptying of the reservoir 10, the flow rate Q _OUT imposed on the pump 19 then being chosen so that the difference between the flow rate of substitution liquid and the flow rate of spent liquid, measured by the balance 23, is equal to the sum of the flow rate Q _WL of desired weight loss and Q _HCO3 infusion rate of bicarbonate solution, measured by the balance 24. The adjustment of the bicarbonate solution infusion pump 22 is carried out as previously according to the control described by formula (1).

In hemodialysis mode, the clamp 15 is closed, the clamp 16 is open and the three pumps 12, 19, 22 rotate. The control unit 25 continuously adjusts the flow rate Q _OUT of the extraction pump 19, so that the difference between the flow rate of dialysis liquid and the flow rate of spent liquid, measured by the balance 23, is permanently equal at the flow rate _{H HCO3} of infusion of bicarbonate solution, measured by the balance 24, the weight loss flow rate instruction being zero.

ce qui suppose une détermination préalable de la clairance du rein artificiel, laquelle dépend du type d'échangeur utilisé (nature de la membrane, surface) ainsi que, en général, des débits du sang et du liquide de dialyse dans l'échangeur.The control unit 25 furthermore regulates the rate Q _HCO3 of infusion of the bicarbonate solution, as a function of the desired concentration of bicarbonate [HCO₃] _DES in the patient's blood, of the concentration [HCO₃] _SOL of the solution contained in the reservoir 20, and of the diffusive loss in the exchanger 1, which is equal to Cl x [HCO₃] _BLD , [HCO₃] _BLD being the concentration of bicarbonate in the patient's blood and Cl, the clearance of the artificial kidney for bicarbonate (clearance is generally defined as the ratio between the quantity of substance eliminated per unit of time and the concentration of the substance in the blood at the entrance to the exchanger). To obtain that the bicarbonate concentration of the blood reaches, at equilibrium, a given value [HCO₃] _DES , it is therefore necessary to control the Q _HCO3 perfusion rate of the pump 22 of bicarbonate solution according to the formula: $${\text{Q}}_{\text{HCO3}}\text{= Cl x}\frac{{\text{[HCO₃]}}_{\text{OF}}}{{\text{[HCO₃]}}_{\text{GROUND}}}$$

which presupposes a prior determination of the clearance of the artificial kidney, which depends on the type of exchanger used (nature of the membrane, surface) as well as, in general, flows of blood and dialysis fluid in the exchanger.

However, for certain values of blood flow and dialysis fluid, the clearance of the kidney for a given substance and type of exchanger is substantially constant. This is particularly the case when, on the one hand, the surface of the membrane of the exchanger is sufficiently large with respect to the blood flow, and when, on the other hand, the blood flow is relatively large with respect to at the flow rate of the dialysis liquid (ratio of the order of three or more). The blood and the dialysis liquid then have, at the outlet of the exchanger, the same concentration in the substance considered, and the clearance C1 is equal to the outlet flow rate of the used liquid, Q _OUT . In other words, under these particular operating conditions, the control of the bicarbonate solution infusion pump 22 is defined by formula (1). However, these conditions are applicable to the treatment by continuous dialysis of patients in shock whose purification must be done at a moderate rate to be able to be supported by weakened organisms.

The artificial kidney according to the invention is therefore of particular interest for the treatment of patients who have lost the temporary use of their kidneys, since, whatever the type of treatment to which they are subjected, this artificial kidney allows, by commanding '' a single pump according to a unique servo formula, to act simply on their acid-base balance.

This artificial kidney can also function in hemodiafiltration mode, in which the position of the clamps and the functioning of the pumps are the same as those which are adopted in hemodialysis mode, except that the pump 19 is controlled for cause ultrafiltration in the kidney in accordance with a given weight loss rate instruction.

The artificial kidney shown in Figure 2 differs from that just described in that its circuit for extracorporeal blood circulation comprises a second pump 26 disposed downstream of the exchanger 1, to allow to vary the transmembrane pressure in exchanger 1 and, consequently, the flow of ultrafiltered plasma water (that is to say Q _OUT in hemofiltration). Furthermore, the reservoirs 10, 17 of dialysis / substitution liquid and of spent liquid are weighed by independent scales 27, 28 and the pipe 18 connecting the compartment 3 of the exchanger 1 to the spent liquid reservoir 17 has no pump .

In addition, a three-way valve 29, to which the pipes 11, 13 and 14 are connected, makes it possible either to connect the reservoir 10 for dialysis / substitution liquid to the bubble trap 8, or to connect the reservoir 10 to compartment 3 of the exchanger 1, that is to isolate the tank 10.

The operation of this second embodiment of artificial kidney according to the invention does not differ significantly from that of the previous one. In hemofiltration mode without infusion of substitution liquid, the pump 12 does not rotate and the flow rate of the pump 26 is adjusted by the control unit 25 so that the filtration flow rate Q _OUT , measured by the balance 28, is equal to the sum of the target weight loss rate Q _WL and the infusion rate of the bicarbonate solution Q _HCO3 measured by the balance 24.

In hemofiltration mode with infusion of substitution liquid, the pump 12 rotates at a flow rate initially set by the operator and the flow rate of the pump 26 is controlled by the control unit 25 to the filtration rate Q _OUT is equal to the sum of the target weight loss rate Q _WL , the infusion rate of the bicarbonate solution Q _HCO3 and the infusion rate Q _IN of the substitution liquid, measured by the balance 27.

In dialysis mode, the pumps 6 and 26 arranged on the blood circuit, respectively upstream and downstream of the exchanger 1, rotate at the same flow rate and the pump 12, which then serves as a circulation pump for dialysis liquid, rotates at a rate initially set by the operator.

In hemodialfiltration mode, the control unit 25 regulates the flow rate of the pump 26 as in hemofiltration mode with infusion of substitution liquid.

Except in hemofiltration mode, where it does not rotate, the flow rate of the dialysis / substitution liquid circulation pump 12 is regulated by the control unit 25 which compares the desired flow rate initially stored in this unit and the measured flow rate by means of the balance 27. As for the bicarbonate perfusion pump 22, its flow rate Q _HCO3 is, as before, controlled by the flow of spent liquid Q _OUT , measured by the balance 28, in accordance with formula (1) and, where appropriate, to formula (2).

The invention is not limited to the embodiments which have just been described and it is susceptible of variants.

In particular, unlike what is provided in the examples of artificial kidney described above, it is possible to provide that, in the modes where dialysis / substitution liquid is circulated by the pump 12, it is the flow rate of the pump 19 (26) controlling the ultrafiltration flow rate which is initially set by the operator and the flow rate of the pump 12 which is controlled as a function of the balance of fresh and used liquids, perfused and ultrafiltered and the desired weight loss rate.

Furthermore, the determination of the flow rate of the liquids necessary for the control of the pump 19 (26) regulating the ultrafiltration flow rate and for the control of the bicarbonate solution infusion pump 22 could be done with other measuring means as scales, for example with flow meters or volumetric measuring means.

In addition, the pumps 12, 22 used to regulate the flow rate of dialysis / substitution liquids and bicarbonate solution could be replaced by electromagnetic clamps, the liquids flowing by gravity.

It also goes without saying that the source 20 of perfusion liquid could be connected directly to the vascular system of the patient 9 and not to the circuit 5, 7 for extracorporeal circulation of blood.

Finally, as mentioned above, the dosing means with which the artificial kidney according to the invention is equipped can be used to dose all kinds of substances in the blood of a patient subjected to a treatment session by hemofiltration, hemodialysis or hemodialfiltration. In the case of a medicament A, for example, the reservoir 20 then contains a sterile solution of this medicament, while the reservoir 10 contains a dialysis liquid in which the main electrolytes of the blood, including bicarbonate, are present. The functioning of the kidney is not different from that which has been described in connection with the exemplary embodiments of FIGS. 1 and 2, and, in particular, the slaving of the perfusion pump 22 to the flow of used liquid is done according to the formula (1), or, where appropriate, according to formula (2).

Claims (15)

1) Artificial kidney comprising: - an exchanger (1) having two compartments (2, 3) separated by a semi-permeable membrane (4), a first compartment (2) being connected to a circuit (5, 7) for extracorporeal circulation of blood connectable to a patient (9), a second compartment (3) having an outlet for the discharge of a used liquid, - means (20, 21) for infusing the patient (9) with an infusion liquid containing a substance (A), characterized in that it comprises metering means (22, 25) for adjusting the concentration of the substance (A) in the patient's blood (9) to a desired concentration [A] _DES , taking into account the diffusive transfer and / or convective of the substance (A) through the membrane (4) of the exchanger (1). 2) artificial kidney according to claim 1, characterized in that the metering means comprise means (22, 25) for adjusting the flow rate of the perfusion liquid (Q _A ) according to the flow rate (Q _OUT ) of used liquid. 3) Artificial kidney according to claim 2, characterized in that the flow rate (Q _A ) of the perfusion liquid and the flow rate (Q _OUT ) of the used liquid are linked by the formula: $${\text{Q}}_{\text{AT}}\text{=}\frac{{\text{[AT]}}_{\text{OF}}}{{\text{[AT]}}_{\text{GROUND}}}{\text{x Q}}_{\text{OUT}}$$

where [A] _SOL is the concentration of substance (A) in the infusion fluid. 4) Artificial kidney according to claim 2, characterized in that the flow rate (Q _A ) of the perfusion liquid and the flow rate (Q _OUT ) of the used liquid are linked by the formula: $${\text{Q}}_{\text{AT}}\text{=}\frac{{\text{[AT]}}_{\text{OF}}}{{\text{[AT]}}_{\text{GROUND}}}\text{x Cl}$$

where [A] _SOL is the concentration of substance (A) in the infusion fluid and Cl is the clearance of the artificial kidney for substance (A). 5) Artificial kidney according to one of claims 1 to 4, characterized in that the perfusion means comprise a source of liquid (20) connected by a pipe (21) to the circuit (5, 7) for extracorporeal circulation of blood. 6) Artificial kidney according to one of claims 2 to 5, characterized in that the means for adjusting the flow rate of the perfusion liquid comprise a flow control member (22) and a control unit (25) for controlling the flow control (22). 7) artificial kidney according to claims 5 and 6, characterized in that the flow control member is a pump (22) disposed on the pipeline (21). 8) Artificial kidney according to one of claims 1 to 7, characterized in that the substance (A) is bicarbonate. 9) artificial kidney according to claim 8, characterized in that it comprises a source (10) of substitution liquid not containing of bicarbonate connected to the circuit (5, 7) for extracorporeal circulation of blood. 10) Artificial kidney according to one of claims 8 and 9, characterized in that it comprises a source (10) of dialysis liquid not containing bicarbonate connected to an inlet of the second compartment (3) of the exchanger (1) . 11) Artificial kidney according to claim 8, characterized in that it comprises a source (10) of dialysis / substitution liquid not containing bicarbonate connected to the circuit (5, 7) for extracorporeal circulation of blood and to an inlet of the second compartment (3) of the exchanger (1), and occlusion means (15, 16; 29) for either isolating the source (10), or allowing the flow of dialysis / substitution liquid in the circuit (5, 7) for extracorporeal circulation of blood, or else allow the flow of dialysis / substitution liquid in the second compartment (3) of the exchanger (1). 12) Artificial kidney according to one of claims 1 to 11, characterized in that it comprises means (19, 26) for varying the ultrafiltration flow rate in the exchanger (1). 13) Artificial kidney according to one of claims 1 to 12, characterized in that it comprises means (23, 24, 25; 24, 27, 28, 25) for taking stock of the liquid (s) introduced. (s) in the circuit (5, 7) for extracorporeal circulation of blood and used liquid (ultrafiltrate and / or spent dialysis liquid) flowing from the second compartment (3) of the exchanger (1). 14) Artificial kidney according to claim 13, characterized in that the means for making the balance of liquids comprise a balance (23) for weighing a reservoir (10) constituting a source of dialysis / substitution liquid and a reservoir (17) for collecting the used liquid, and a balance (24) for placing a reservoir (20) constituting a source of liquid containing the substance (A). 15) Artificial kidney according to one of claims 12 and 13, characterized in that it comprises means (25) for controlling the means (19, 26) for varying the ultrafiltration rate according to the balance of liquids and a given weight loss flow instruction (Q _WL ).

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